Salt is one of the most abundant materials on earth and is one of the largest volume inorganic materials used in industry in the production of chlorine and caustic soda by the electrolytic processes and is used in the manufacture of many products, both organic and inorganic. Salt is also used for snow and ice control and as a mineral in animal diets, as a food preservative and for flavoring food. Salt is the most common and readily available non-metallic mineral in the world. Oceans contain an inexaustible supply of salt. The identified resources of salt in the United States alone are estimated at over sixty trillion tons.
Salt is produced by direct removal as rock salt from underground deposits, by solution mining underground deposits, or by evaporation from solution mining or naturally occuring brines or sea water. Underground deposits are large beds of concentrated salt which have been deposited through evaporation of brines over the geological ages. Underground deposits are mined as rock salt using conventional mining techniques or by solution mining creating a brine. In solution mining water is pumped into the salt bed, the salt is dissolved in the water and the resultant brine is brought to the surface. Many processes and techniques have been disclosed for the mining and production of salt from these various raw material sources, and many processes and techniques have been disclosed for the purification of the salt produced by the mining processes. U.S. Pat. Nos. 3,647,396 and 3,655,333 are examples of disclosures of processes for purifying salt already produced.
Processes have been employed and described in the prior art for the production of high purity salt at the initial site where the salt recovery processes are used for the mining of the salt. Because of the high cost of energy, especially in the cost of petroleum derived energy, created by the changes in the mid-east two decades ago, which created the energy crisis, many attempts have been made to optimize the conservation of energy in the production of high quality salt. Background information on the processes, equipment and techniques employed in these endevours are described in the Encyclopedia of Chemical Technology, edited by Kirk-Othmer, Third Edition, Volume 9, under the heading Energy Management starting on page 21 through 45, and under the heading Evaporation, starting on page 472 through 493. Additional background information is also disclosed in the Encyclopedia of Chemical Processing and Design, edited by John J. McKetta, Volume 20, under the heading Evaporator Operation starting on page 396 continuing under the heading Evaporation through page 445. Perry's Chemical Engineers' Handbook, Sixth Edition, under Evaporators, starting on page 11-31 through 11-43 also provides background information related to this invention.
A recent process design for the production of evaporative salt from solution mined brine which pursues the objectives of making salt while at the same time conserving the use of energy is described in the publication of the Fifth International Symposium on Salt--Northern Ohio Geological Society in an article by A. Pavik, G. Arcangeli and J. C. Gallot, starting on page 335 thru 339. The article describes a process installed by Montedison at Ciro Marina-Calbria, Italy. The article describes a salt plant with solution mining and an evaporation plant employing quadruple effect evaporators and a mechanical recompression evaporator and includes the generation of steam at high pressure which is used to drive two steam turbines. One of these steam turbines is connected to an alternator which generates the necessary electric current used in the plant and the other is used to drive a compressor which recompresses the vapors from the single effect evaporator, so that it can be reused in the heating elements of the single effect evaporator. The excess steam from both steam turbines is used to drive the quadruple effect evaporator train.
In accordance with this invention, we employ an evaporative salt plant, including methods of operation and apparatus which produce high purity salt economically and in high yield, comprising the combination of a gas turbine which drives a vapor compressor while the gas turbine exhaust gases are used to produce high pressure steam which is used to drive a steam turbine, which in turn generates the electrical energy requirements of the plant, and wherein the discharge vapors from the steam turbine are combined with the discharge vapors from the vapor compressor, which is in turn in combination with a vapor compression evaporator and a purge evaporator, whereby both evaporators produce salt, and where the overhead vapors of the purge evaporator are used in a brine cooled condenser to preheat input cold brine. Water condensate is recovered from the evaporator heater and brine cooled condenser and used in solution mining the underground salt, thereby allowing for productive use and recovery of substantially all the raw material and over 70% of energy inputs to the plant, and friendly environmental operation of the plant.